Coherent line-cards using polarization-multiplexed quadrature phase-shift keying (PM-QPSK) or polarization-multiplexed quadrature amplitude modulation (PM-QAM) has become de-facto standards in quest for high spectral efficiency optical fiber communications systems. When a modulated signal passes through multiple reconfigurable optical add drop multiplexers (ROADMs), the bandwidth of the propagating signal usually narrows down due to the spectral shape of the ROADM filter and due to possible misalignments of the central frequencies of the signal and the ROADM pass-band. This bandwidth narrowing can in turn increase the observed bit error ratio (BER) at the receiving end and cause the receiver to lose track of the signal. Most dynamic gain equalizers (DGEs) can also introduce bandwidth narrowing by flattening the gain spectrum of cascaded Erbium-doped fiber amplifiers (EDFAs). The bandwidth narrowing effect due to ROADMs and DGEs is relatively dynamic.
In addition, photodiode, trans-impedance amplifier, circuit traces, connector for pluggable optics, and analog digital converters (ADCs) form an analog interface between coherent receiver and digital signal processing (DSP) chip. There is also a similar analog interface between the DSP chip and the coherent transmitter. Both analog interfaces can influence the signal bandwidth. Although this influence can be relatively static, it might degrade over the lifetime. Accordingly, a need exists for methods and apparatus that compensate for the bandwidth narrowing effect.